Semiconductor fabrication facilities utilize clean rooms for various chip manufacturing operations. Because of the nature of the manufacturing operations, it is usually a requirement that a very precise and constant rate of purified air be supplied to any given clean room. The purified air is fan-driven and purified by one or more filters (generally HEPA filters are used, which are well-known). The manufacturing operations may be adversely affected if a constant volumetric air rate is not maintained.
Over time, the filters become increasingly clogged or loaded with particulate matter which impedes the air rate delivered to the clean room. Filter loading creates a need to make compensating adjustments in order to maintain a constant air rate.
In the past, it was commonplace to use fixed-speed fans to deliver the air. Air adjustments were made by moving or repositioning barriers in the duct work or air path leading into the clean room. These barriers were usually in the form of screens or dampers and functioned to alter the cross-sectional area of the air path. The result was a change in air velocity and rate. With this adjustment method, however, it is difficult to precisely adjust the air.
Alternatively, the air may be adjusted by using a variable frequency drive ("VFD") to control the fan. A VFD adjusts the frequency of the input power to the fan motor, which adjusts the fan speed, thereby adjusting the volumetric air rate. The higher or lower the fan speed, the higher or lower the air rate, respectively.
When VFDs are used in conjunction with clean room air, the clean room operator must repeatedly measure how much the actual volumetric air rate has deviated from a pre-set standard and make corresponding adjustments to fan speed. In the past, the operator made the air rate measurement by using a hand-held measuring device. Then, depending on the measurement, the operator gradually adjusted fan speed until the air rate returned to the pre-set standard. This was, of course, labor-intensive, but it also had a significant drawback in that the air rate could drop out of adjustment a significant amount before the operator detected it via a manual measurement. If the air rate changed too much before detection, it could have an adverse impact on clean room manufacturing operations. The present invention solves these problems.